1. Field of the Invention
The present invention relates to terrestrial cellular mobile communication systems. More specifically, the present invention relates to a method and apparatus for controlling transmission power in a code division multiple access (CDMA) cellular mobile communication system.
2. Description of the Related Art
Transmission power control using CDMA techniques in terrestrial cellular mobile communications is disclosed, for example, in International Publication No. WO91/07037, and Japanese Unexamined Patent Publication No. 7-283783. Such a CDMA-based transmission power control, however, poses a problem of distance variation among mobile units using the same base station in the cellular mobile communication system. In such a situation that one mobile unit may be located at a quite long distance from the base station while another mobile unit may be located very near, the base station receives signal power at a higher level from the mobile unit very near. Signal power at such a higher level received from a shorter distance interferes with the base station receiving signal power from other mobile units located at longer distances. A more sophisticated control technique of transmission power is urgently needed for realizing the equal level of transmission power received at the base station from each one of mobile units sharing the same base station in the cellular mobile communication system, which is essential to solve the distance variation problem and to increase in-system user capacity.
The CDMA cellular mobile communication system poses another problem of system interference. When the base station controls the transmission power of a mobile unit during communication so that the base station can receive signal power from other mobile units at an equal level, nevertheless, interference occurs due to the common frequency assigned to all mobile users in the communication system sharing the same base station with other users who may be communicating at the same time. Interference is closely related to the quality of communication in cellular mobile communication system. In order to avoid such interference, the base station tries to transmit power to each mobile unit at a minimum level required for maintaining quality communication, in terms of increasing the system user capacity. Such a CDMA-based transmission power control involves an open loop control system through the local assumption of transmission power at each mobile unit and a closed loop control system through a command from the base station for adjusting and fine tuning the transmission power at the mobile unit.
FIG. 29 shows a block diagram of the related art transmission power control apparatus, incorporated in a closed loop control system of transmission power between a base station 101 and a mobile unit 102 in the CDMA cellular mobile communication system. At the mobile unit 102, transmission power is controlled locally through the open loop control system, which is not shown here. The base station 101 generates a transmission power control command signal, which is abbreviated to TPC bit, based upon transmission power received from the mobile unit 102 and transmits the TPC bit to the mobile unit. The transmission power received at the base station is converted to Signal-to-Interference Ratio, which is abbreviated to Receive (REC.) SIR, through conventional conversion techniques which are not shown or discussed here.
The TPC bit is generated at the base station 101 as follows. An adder 103 subtracts the Receive SIR (in dB) from a predetermined Reference Signal-to-Interference Ratio REF. SIR (in dB) to calculate a difference as a power control error .epsilon. (in dB) and outputs the power control error .epsilon. to a judger 104. The judger 104 judges a value (.+-.) corresponding to the sign of the power control error .epsilon. and generates the value as the TPC bit. When Reference (REF.) SIR&gt;Receive (REC.) SIR, for example, the judger 104 outputs a TPC-bit value (+1) requesting the mobile unit 102 to raise the power level for transmission. When Reference SIR.ltoreq.Receive SIR, the TPC-bit value (-1) is outputted requesting to lower the transmission power level. The TPC bit is a one-bit command signal (.+-.1) having the significance in the sign (.+-.). The one-bit command signal of the TPC bit is transmitted through a multiplex technique on a traffic channel to a mobile unit at a transmission rate of some hundreds to thousands bits per second. The values of TPC bit, +1 and -1, are in fact converted to 0 and 1, respectively, for transmission.
Upon reception of the TPC bit from the base station 101 through the closed loop control system, the mobile unit 102 increase or reduces transmission power based upon the TPC bit using a predetermined fixed step size .DELTA. (in dB), 1 dB for example. Specifically, the TPC bit multiplexed on a traffic channel is extracted. The TPC-bit value (.+-.1) is amplified or multiplied by .DELTA. by an amplifier 105 to become +.DELTA. (in dB) with a command signal (+1) and -.DELTA. (in dB) with a command signal (-1). The amplified result is integrated in an integration circuit including an adder 106 and a delay circuit 107 with previous values of the TPC bit. With a series of values of TPC bits like (+1, +1, +1, +1, -1, +1, -1, -1) when an initial value 0 (in dB) is assigned to the delay circuit 107, the adder 106 outputs an integrated result of (+.DELTA., +2.DELTA., +3.DELTA., +4.DELTA., +3.DELTA., +4.DELTA., +3.DELTA., +2.DELTA.). The integrated results of controlled power through the closed loop control system are added to optimal power assumed in the open loop control system, not shown. Thus, the mobile unit 102 transmits adjusted power through the closed and open loop systems to the base station 101.
Power control delay may occur in the closed loop control system depending upon the location of the mobile unit 102 including the delay resulting from the TPC bit travelling from the base station 101 to the mobile unit 102, the TPC bit adjusting transmission power, and the adjusted transmission power travelling from the mobile unit 102 to the base station 101.
FIG. 30 shows a graph of a power variation characteristic of transmission power received at the base station 101, illustrating a desired type of power variation involving no power control delay in the closed loop control system. The graph is drawn with a time variation t as the horizontal scale indicating each periodical term of TPC bit to be outputted, abbreviated to a TPC period T, and with dB of Receive SIR at the base station 101 of transmission power from the mobile unit 102 as the vertical scale indicating each step size .DELTA. when the value of Reference SIR is set to zero. Referring to the graph, TPC bits 501 through 504 are generated, respectively, based upon Receive SIR values (in dB) 401 through 404 at the base station 101. The values of Receive SIR remain within a desired range of .+-..DELTA. with the reference value zero when transmission power varies within 1.DELTA. in a single TPC period of 1T. Specifically, with Receive SIR 401&lt;0, a value (+1) is assigned to TPC bit 501, whereby Receive SIR 402 is increased by approximately .DELTA. from Receive SIR 401. With Receive SIR 402.gtoreq.0, a value (-) is assigned to TPC bit 502, whereby Receive SIR 403 is reduced by approximately .DELTA. from Receive SIR 402. In such a manner, transmission power of a mobile unit is adjusted desirably based upon a TPC bit when involving no power control delay.
FIG. 31 shows a graph of a power variation characteristic of transmission power received at the base station involving the power control delay of 1T, illustrating poor performance of transmission power control in the same graph structure as that of FIG. 30. According to the graph of FIG. 31 involving the power control delay of 1T, the values of Receive SIR range between .+-.2.DELTA. with the reference value zero when transmission power moves only within 1.DELTA. in a single TPC period of 1T. This illustrates that there is a time lag of 1T between a generated and transmitted TPC bit and Receive SIR of transmission power to be adjusted based upon the TPC bit due to the power control delay of 1T in the closed loop control system. Referring to the graph, TPC bits 511 through 514 are generated based upon Receive SIR values 411 through 414. Specifically, with Receive SIR 411&lt;0, a value (+1) is assigned to TPC bit 511. With a value (+1) of the previous TPC bit 510, Receive SIR 412 is increased by approximately .DELTA. from Receive SIR 411. With Receive SIR 412.gtoreq.0, a value (-1) is assigned to TPC bit 512. With the previous TPC bit 511 (+1), Receive SIR 413 is increased by approximately .DELTA. from Receive SIR 412. With receive 413.gtoreq.0, a value (-1) is assigned to TPC bit 513. With the previous TPC 512 (-1), Receive SIR 414 is reduced by approximately .DELTA. from Receive SIR 413. Transmission power of the mobile unit is adjusted in such a poor manner resulting in transmission power ranging .+-.2.DELTA. with the power control delay of 1T.
Thus, the power control delay in the closed loop control system delays the TPC bit to adjust transmission power at the mobile unit, which ends up in poor power control. With the power control delay of 2T or more, therefore, the transmission power range will be much greater.
The transmission power control apparatus of FIG. 29, therefore, poses some problems as follows. With received channel power level changing little, nevertheless, transmission power received at the base station varies greatly by a parasitic effect due to a power control delay of more than 1T. Consequently, power control error becomes greater, which results in reduced system user capacity.
Another problem concerns the fixed step size .DELTA. for power adjustment. The fixed step size may not be suitable in some cases depending upon the motion speed of a mobile unit. Power control error may increase which also leads to reduced system user capacity.
FIG. 32 shows a graph of control error characteristics of transmission power involving the power control delay of 1T, illustrating an fDT as the horizontal scale and a standard deviation .sigma. of transmission power control error as the vertical scale. With fDT, fD stands for a Doppler frequency and T stands for the TPC period. Doppler frequency corresponds to the motion speed of a mobile unit. With transmit frequency in the band of 1 GHz and a motion speed of the mobile unit at 100 km/h, for example, Doppler frequency fD is 90 Hz. Therefore, fDT=0.1 with a single TPC period T of 1.1 kbps, for example. The graph includes a characteristic curve A of control error having a step size .DELTA.1 and a characteristic curve B of control error having a step size .DELTA.2. Step size .DELTA.1 is smaller than step size .DELTA.2 in such a relation, for example, that .DELTA.2=2 dB when .DELTA.1=1 dB. There is a crossing point of characteristic curves A (.DELTA.1=1 dB) and B (.DELTA.2=2 dB) when fDT=0.01. This indicates that characteristic A (.DELTA.1=1 dB) has less control error of transmission power when fDT.ltoreq.0.01, and characteristic B (.DELTA.=.DELTA.2 dB) has less control error of transmission power when fDT&gt;0.01.
Still another problem exists concerning the transmission power control command. The transmission power control apparatus proposes another type of control command signal using a multi-bit command signal as an alternative of the single bit control command signal of the TPC bit. A multi-bit signal transmission, however, poses another problem concerning the greater number of TPC bits in transmission rate required from the base station to each mobile unit.
Yet another problem concerns system interference. The fixed step size .DELTA. is the only way to account for a possible sudden and sharp movement in channel power. The step size is not appropriate enough to control such an emergency situation and might leave interference out of control.
FIG. 33 shows a graph of a power variation characteristic of transmission power received at the base station involving a power control delay of 1T. The graph is drawn with time variation t as the horizontal scale and with dB of the Receive SIR of transmission power from the mobile unit received at the base station. Referring to FIG. 33, a fading-related fall and a great system interference occurring in Receive SIR designated by a solid line 602 is shown as a result of the poor control of transmission power from a mobile unit of a great increase in channel power designated by a broken line 601 of transmission power. Troublesome system interference may occur when a change in channel power goes far beyond power control based upon Receive SIR at the base station. Specifically, system interference occurs in the following manner with FIG. 33. With a great fall in Receive SIR due to the fading effect, the base station continuously generates an increase command (+1) of the TPC bit in order to increase transmission power of the mobile unit to recover the normal power. As a result, when Receive SIR recovers from the fading-related fall and transmission power reaches a high enough power level, nonetheless, the value of transmission power does not come down to a predetermined value or below. This is caused by the step size .DELTA. being too small to cope with such a change in transmission power and by the power control delay.